The present invention relates to a paper feeding unit provided in an image recording apparatus such as a copier, facsimile or printer, and more particularly relates to improvements in a paper feeding unit which feeds papers sheet by sheet from a paper feeding cassette or a hand feeding tray in which a plurality of papers can be held.
A paper feeding unit which feeds a sheet-shaped paper (such as a recording paper, a document paper or the like) sheet by sheet from the top of a stack of papers placed on a paper tray, paper feeding cassette or paper stacker, has been conventionally provided in an image recording apparatus such as a copier or printer. A corner separating system in which a separating claw is used and a frictional separating system in which a roller and belt are used, are commonly put into practical use for an upper paper feeding cassette.
A paper feeding unit provided with a corner separator (a separating claw), a bottom plate pushing member oscillated by a cam, a movable bottom plate, and a paper feeding roller, has been disclosed in the official gazette of Japanese Patent Application Open to Public Inspection No. 135034/1980.
A paper feeding unit provided with a corner separator, a coil spring, and a paper feeding roller, has been disclosed in the official gazette of Japanese Patent Application Open to Public Inspection No. 46255/1981.
A paper feeding unit of a frictional separation system has Application Open to Public Inspection No. 127935/1988 and No. 123772/1989, and in the official gazettes of Japanese Utility Model Open to Public Inspection No. 96441/1989 and 121036/1989. In this system, a separation claw and coil spring are not provided inside the paper feeding cassette, so that recording papers can be easily loaded and exchanged as compared with the corner separating system.
A pick-up roller which comes into contact with stacked papers, and a feed roller and reverse roller to prevent double feeding are provided in the position close to the leading edges of the papers placed in a paper feeding cassette in the paper feeding unit of a frictional separation system.
In the paper feeding process of this paper feeding unit, a paper is fed as follows. First of all, the upper surface of the paper is raised up to the set position, and the rotation of a pick-up roller and feed roller is started in accordance with a paper feeding signal. The pick-up roller which comes into contact with the upper surface of the paper with pressure, conveys the paper to a nip position between the feed roller and reverse roller. The reverse roller is driven in the direction reverse to the paper advance, and contacted with the feed roller with pressure by a spring force.
When the reverse roller is directly contacted with the feed roller (in other words, when there is no paper in the nip position), or when a sheet of paper is fed into the nip position, the torque given to the reverse roller exceeds the limit value so that the reverse roller slips and rotates together with the feed roller. However, when not less than 2 sheets of papers are fed into the nip position, the limit torque becomes stronger than the frictional force between the papers so that the reverse roller rotates in the opposite direction. Accordingly, the lower side paper in the nip position is pushed back so that double feeding (which is to feed a plurality of papers simultaneously) can be prevented. In the manner described above, papers are separated sheet by sheet and conveyed to a register roller.
Conventionally, there are two types of paper cassette loading system, one is a side loading type and the other is a front loading type.
In the side loading type, a cassette insert opening is formed on one side of the recording apparatus body, and a paper feeding cassette is set to the opening laterally. The papers in the paper feeding cassette are conveyed by a paper feeding unit in the same direction as the paper feeding cassette inserting direction.
As described above, in the aforementioned side loading type, the paper cassette inserting direction is the same as the recording paper feeding direction, so that the relation between the paper feeding unit and the leading edge position of the recording papers is determined when the paper feeding cassette is set to the cassette insertion opening. Accordingly, this type of paper feeding cassette loading system is advantageous in that: a recording paper is easily conveyed; and when there is a jam in recording paper conveyance, the jammed paper can be easily removed from the apparatus by pulling out the paper feeding cassette.
On the other hand, this type of paper feeding cassette loading system has the following shortcomings: a space must be made on a side of the recording apparatus in order to detach the paper feeding cassette from the apparatus so that a wide space of installation is needed; and the maneuverability of the apparatus is not so good since the paper feeding cassette is handled on the side of the recording apparatus.
The front loading type of paper feeding cassette loading system is composed in such a manner that: a cassette feed opening or a cassette stacker is provided on the working side of the image recording apparatus and the paper feeding cassette is inserted into the apparatus from the working side; and a recording paper is fed in the direction perpendicular to the paper feeding cassette insert direction (which has been disclosed in the official gazettes of Japanese Patent Application Open to Public Inspection Nos. 45466/1984 and 50062/1987).
According to the structure described above, the paper feeding cassette can be detached on the working side of the apparatus, so that the following advantages can be provided: the maneuverability is high; and the space of installation is small since there is no need to leave a space on the side of the apparatus.
In the aforementioned conventional frictional separation type of paper feeding mechanism, after the first paper has been conveyed by the feed and reverse rollers, the leading edge of the second paper is pushed back by the reverse roller and stays in the position close to the nip position of the above-described two rollers coming into contact with the circumferential surface of the reverse roller.
If the paper feeding cassette is pulled out under the condition described above, the uppermost paper remains in the recording paper feeding unit. When an operator attaches the recording paper feeding cassette, and he is not aware of the residual paper, and a paper jam will occur causing the sensor and roller to damage.
Further, the following problem occurs. When the uppermost paper of the paper feeding cassette is located in the position close to the reverse roller and another paper is fed by a hand feeding unit, the paper is overlapped onto the previous paper so that a paper jam occurs.
In the case of the front loading type of paper feeding unit, the recording paper is conveyed in the direction perpendicular to the paper feeding cassette inserting direction. Accordingly, when the recording operation is stopped, several uppermost recording papers housed in the paper feeding cassette are pushed out in the paper feeding direction by the aforementioned pick-up roller, so that the leading edges of the pushed-out recording papers are located in the position closed to the nip position between the feed and reverse rollers. If the paper feeding cassette is pulled out under the condition described above, the recording papers are pulled out in the direction perpendicular to the paper feeding direction while the leading edges of the several uppermost papers are protruded into the position close to the nip position and the stack of papers in the paper feeding cassette are pressed by the pick-up roller. Consequently, the leading edge portions of the several uppermost recording papers remain inside the recording apparatus and the middle portions of the papers are pulled out from the apparatus, so that the recording papers are stretched and torn.